1. Field of the Invention
The present invention relates to a holographic recording method. Particularly, the invention relates to the holographic recording method which performs multiplex recording of data into an optical recording medium in the form of a hologram.
2. Description of the Related Art
A holographic memory receives attention as a computer memory of the next generation. The holographic memory has both large capacity derived from a three-dimensional recording region and high speed derived from a two-dimensional batch recording and reproducing method. In the holographic memory, a plurality of data pages can be recorded by multiplexing the data pages in the same volume, and the data can be read out in each page by one operation. The digital data can be also recorded and reproduced in such a manner that not an analog image but binary digital data “0,1” is digitized in the form of “bright and dark” and recorded and reproduced in the form of the hologram. In recent years, a specific optical system of this digital holographic memory system, an S/N ratio or a bit error rate evaluation based on a volume multiplexing method, or two-dimensional coding has been proposed. Also, studies proceed from a more optical point of view such as influence of aberration of the optical system.
A shift multiplexing method which is one of the volume multiplexing methods will be described referring to FIGS. 11A and 11B. In the shift multiplexing method, a light wave whose wavefront is rapidly changed, such as a spherical wave and a speckle pattern, is used as a reference light beam. In the case where the reference light is used, Bragg condition for the reproduction can be avoided only by slightly shifting a position of the recording medium by the amount of shift δ from the recording spot (FIG. 11B), and the new hologram can be recorded there. That is to say, the multiplexing of the hologram can be performed in the substantially same volume by slightly shifting the recording medium. As described above, in the digital holographic storage, both the high-speed transfer by the two-dimensional batch recording and reproducing method and the increase in the recording capacity by the volume recording can be realized at the same time.
In the case where the multiplexing is performed, in order to sufficiently use a dynamic range of the optical recording medium, there is proposed a schedule exposure method in which exposure time is gradually decreased from a front-end page to a final page so that a diffraction efficiency of each page to be multiplexed becomes equal after the recording is performed so that the diffraction efficiency of the front-end page becomes maximum (Psaltis, et al, Applied Optics vol. 27, issue 9, page 1752 (1988)).
Usually the digital data is expressed in the form of a two-dimensional image, when the digital data to be recorded in the recording medium is digitally displayed by a spatial light modulator. In the two-dimensional image, the maximum step of gradation is indicated as 1 of the digital data and the minimum step of the gradation is indicated as 0 of the digital data. The maximum step of the gradation can be indicated as 0 of the digital data and the minimum step of the gradation can be indicated as 1 of the digital data. The two-dimensional image is transferred to the spatial light modulator to generate a signal light beam, and the two-dimensional image is recorded in the recording medium in the form of the hologram. In reproducing the hologram, a reproduced image is received by using a detector such as CCD. The original digital data is decoded from the reproduced image. For example, in the case where the reproduced image is received by CCD in which each pixel has an 8-bit gradation (256-step gradation), in the 256-step gradation, ideally 1 of the original data is indicated by 255 (white) and 0 of the original data is indicated by 0 (black)
Here, the gradation of each data varies by recording and reproducing conditions, setting of the dynamic range of CCD, noise, or the like. When the gradation of each data is shown by a histogram, there are two distributions of the distribution indicating 1 and the distribution indicating 0 in the histogram. When the distributions are separated from each other, the decoding can be correctly performed in such a manner that the gradation step not belonging to both distributions is used as a threshold to compare the threshold to the gradation step of the data.
As an overlap between the two distributions is increased, the number of pieces of the data in which 1 can not be correctly distinguished from 0 is increased, and readout error is increased. Accordingly, in order to decrease the readout error, it is necessary that the two distributions do not overlap with each other but the two distributions are separated from each other. Further, in order that the threshold used for the decoding is fixed irrespective of the reproduced image, it is desirable that the diffraction efficiency of each hologram is fixed.
The conventional schedule exposure method was predicated on the multiplex recording of the hologram having the acceptable maximum number of the recording medium after performing the recording so that the diffraction efficiency of the front-end page becomes the maximum. In this case, the number of pages (degree of multiplicity) to be multiplexed is fixed, so that only one kind of the schedule exposure method could be used.
Here, A situation where the one kind of the schedule exposure method is used is considered. When the multiplexing of an arbitrary number of pages is performed such that the multiplexing is performed in each file, diffraction efficiencies of the holograms included in files having different degrees of multiplicity do not become equal.
That is to say, in the case where the one kind of the schedule exposure method is used, the diffraction efficiency of the front-end page is caused to be a maximum, and the number of exposure times is determined according to the number of pages to be multiplexed. Therefore, large variations in the final diffraction efficiencies are generated between the case in which the multiplexing of the large number of pages is performed and the case in which the multiplexing of the small number of pages is performed. In this case, the diffraction efficiency of the case in which the multiplexing of the small number of pages is performed is larger than that of the case in which the multiplex recording of the large number of pages is performed. Accordingly, in order that the final diffraction efficiency is caused to be equal irrespective of the degree of multiplicity, it is necessary to adopt the proper exposure schedule according to the degree of multiplicity.